Nonwoven fibrous webs used as filtration media often comprise two or more kinds of fibers, each having a different average diameter so the web can filter particles of a broad range of sizes. Generally the different kinds of fibers lie in different layers of the web. One example taught in Healy, U.S. Patent Application Publication No. US 2004/0035095, is a filtration web comprising a layer of microfibers having diameters between about 0.8 and 1.5 micrometers meltblown onto a spunbond web (see paragraphs [0009] through [0012]). A problem with such a web is that such small microfibers, exposed on the top of the web, are very fragile and easily crushed by normal handling or contact with some object. Also, with very fine diameter fibers it can be difficult, because of the very low weight of an individual fiber, to transport the fibers and retain them in an efficient fiber stream. Some of the very fine fibers tend to scatter as they issue from a meltblowing die rather than travel as a contained stream to a collector.
Another example of prior-art multilayer, multidiameter fibrous webs is so-called SMS webs, comprising a layer of spunbond fibers, a layer of meltblown microfibers, and another layer of spunbond fibers. The multilayer nature of such webs increases their thickness and weight, and also introduces some complexity in manufacture.